The relative roles of abiotic conditions and competition by grassland plants in restricting the distribution of certain alpine plant species to snowbed habitats (i.e. sites with particular long-lasting snow cover) is still a matter of debate. Here, we set up a five-year field experiment to compare three vital rates (germination from sown seeds, growth and survival of transplanted juveniles) of four calcareous alpine snowbed plants within sparsely and densely vegetated plots along a natural snowmelt gradient from the centre of a snowbed to the margins of the surrounding grasslands. Without a dense canopy, we found germination and juvenile growth to respond homogeneously across species with optimal performance under intermediate snowmelt. Dense vegetation cover reduced these two vital rates for all species, with the strongest effects of competition detectable at intermediate snowmelt times again. Survival patterns were less consistent among species. In particular, two of the four species showed lowest mortality at the grassland margins when the surrounding vegetation cover was sparse, and the effect of a dense canopy on their survival turned from competitive to facilitative towards the snowbed centre. Overall, these results indicate that the distribution limits of the studied snowbed plants are determined by a combination of abiotic and biotic effects with competition further reducing their performance in grassland sites below the already low levels caused by the abiotic conditions. With respect to climate warming, a combined negative effect of reduced snow cover protection and increased competition by encroaching grassland plants seems a worrying, but possible scenario. We concede, however, that care has to be taken when drawing conclusions on cause-effect relationships from experiments along natural gradients and discuss the uncertainties associated with our results as well as with inferences on the possible future of snowbed plants under a warmer climate. © 2011 Perspectives in Plant Ecology, Evolution and Systematics.
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